The present invention relates to a battery pack for an electric power tool constituted by connecting multiple chargeable and dischargeable cells in series.
In this type of battery pack for an electric power tool, when the battery is used repetitively up to an over discharge level, a problem occurs in which capacity imbalance progresses among the cells constituting the battery and the battery life is reduced.
In order to avoid this sort of problem, in JP2008154317A, it is suggested to measure a voltage of each cell at a set timing when a voltage of at least one of the multiple cells reaches or falls below a discharge cutoff voltage and to allow each cell to discharge such that the voltage of each cell is controlled to be the minimum voltage out of all the measured voltages.
That is, according to this device suggested, a balancing control is performed to reduce difference in voltages of the respective cells. In details, the cell/cells having a voltage higher than the minimum voltage is controlled to discharge at a predetermined set timing so that the voltage of each cell stays at the minimum voltage. Accordingly, the voltages of all the cells become substantially the same and the difference in voltages among the cells can be reduced.